Epoxy resin compositions each containing an epoxy resin and a curing agent therefor as essential components exhibit excellent heat resistance and insulation when used in cured products, and are thus widely used in application to electronic components such as semiconductors, multilayer printed circuit boards, etc.
In the application to electronic components, in the technical field of multilayer printed circuit board insulating materials, various electronic apparatuses have recently been increasing in speed and frequency of signals. However, it is becoming difficult to achieve low dielectric loss tangent while maintaining sufficiently low dielectric constant with increases in signal speed and frequency.
Therefore, it is desired to provide a thermosetting resin composition capable of producing cured products which exhibit sufficiently low dielectric loss tangent while maintaining sufficiently low dielectric constant for signals at higher speeds and higher frequencies. A technique known to provide materials capable of realizing such low dielectric constant and low dielectric loss tangent uses, as an epoxy resin curing agent, active ester compounds produced by aryl-esterifying phenolic hydroxyl groups in phenol novolac resins (refer to Patent Literature 1).
However, multilayer printed circuit-board insulating materials are required to have very high heat resistance due to the tendency for electronic components to have higher frequencies and smaller sizes, but the active ester compounds produced by aryl-esterifying phenolic hydroxyl groups in phenol novolac resins cause decreases in crosslinking density of cured products due to the aryl ester structures introduced therein, thereby imparting unsatisfactory heat resistance to cured products. Thus, it is difficult to achieve heat resistance and low dielectric constant/low dielectric loss tangent.
On the other hand, it is essential to deal with environmental issues such as the dioxin problem of insulating materials used in the field of semiconductors and multilayer printed circuit boards, and there has been increasing demand for a so-called halogen-free flame retardant system including a resin imparted with a flame retardant effect without using an addition-type halogen-based flame retardant. However, the active ester compounds produced by aryl-esterifying phenolic hydroxyl groups in phenol novolac resins improve dielectric characteristics but degrade flame retardancy of cured products because the compounds contain many combustible pendant-like aromatic hydrocarbon groups in the molecular structures thereof, thereby failing to structure the halogen-free flame retardant system.